The present invention relates to a coil spring that is used in a torsion spring for a torsional vibration absorbing device or the like.
A conventional coil spring such as a torsion spring applied to, for example, a clutch disk is made of a spring wire with a circular section wound in a coil shape. The spring wire is provided with flat faces on a peripheral shape in a cross section thereof as contact faces.
When load is applied to the coil spring until a coil shape is put in a close contact state or in a locked state, each flat face of each coil of the coil spring comes into contact with a flat face of an adjacent coil to receive the load stably and suppress displacement in a diameteral direction of the coil.
In the coil spring, however, stress acting on an inner side portion in the diametral direction of the coil (an inner diameter side portion) is generally higher than stress acting on an outer side portion in the diametral direction of the coil (an outer diameter side portion). In this way, bias of the stress is generated to deteriorate stress dispersion. In addition to the bias, providing the flat faces further deteriorates stress dispersion in a circumferential direction in a cross section of the spring wire.
On the other hand, when the flat faces are provided on the spring wire of the coil spring, smaller oblateness in the cross section of the spring wire is advantageous in designing a low stiffness spring with longer stroke because a close contact length (height) that is a length in an axial direction of the coil spring in the close contact state becomes shorter.
FIG. 13 is a graph showing a relationship between a spring index D/W and a stress ratio due to a difference in oblateness T/W, and FIG. 14 is a graph showing a relationship between a spring index D/W and a close contact height ratio due to difference in oblateness T/W. The symbols of T, W and D are the same as the symbols of a spring wire 101 shown in FIG. 15. Namely, T denotes the maximum thickness in an axial direction of a coil spring, W denotes the maximum width in a radial direction of a coil, and D denotes a coil mean diameter.
In FIG. 13, it shows measurement results in stress ratio obtained from coil springs each having flat faces provided on a spring wire with a circular section in state of fixing spring constant and a close contact height. In FIG. 14, it shows measurement results in close contact height ratio obtained from coil springs each having flat faces provided on a spring wire with a circular section in state of fixing spring constant and stress. In FIGS. 13 and 14, stress change with respect to the spring index D/W at the oblateness T/W=0.92 is set in value of 1, and the stress ratios or the close contact height ratios at the oblateness T/W=0.76 are plotted with reference to the stress change at the oblateness T/W=0.92.
As is apparent from FIGS. 13 and 14, both the stress and the close contact height at the oblateness T/W=0.76 are smaller than those at the oblateness T/W=0.92.
FIGS. 15 and 16 are explanatory diagrams each showing analysis results of stress distribution based upon the finite element method in a cross section of a spring wire of a conventional coil spring. A spring wire 101 is made of a circular base wire with a circular section and has flat faces 103 formed on the base wire by wiredrawing to set oblateness T/W in 0.92. A spring wire 105 is made of a circular base wire with a circular section and has flat faces 107 formed on the base wire by wiredrawing to set oblateness T/W in 0.76.
As is apparent from comparison between FIGS. 15 and 16, by forming the flat faces 103 and 107, the spring wires 101 and 105 each made of the circular base wire with the circular section can disperse stresses at inner diameter side portions 108 up to the flat faces 103 and 107. In this way, stress dispersion in a circumferential direction can be achieved according to reduction of the oblateness T/W. However, continuity of the stress distribution lowers in a circumferential direction, so that forming the flat faces 103 or 107 on the circular base to reduce oblateness prevents evenness of stress.
FIG. 17 shows an analysis result of a stress distribution based upon the infinite element method like FIGS. 15 and 16. A spring wire 109 is made of a rectangular base wire with a rectangular section. Even in a case of the spring wire 109 having the rectangular section, stress acting on an inner diameter side portion 108 can be dispersed and a load can be stably received in a close contact state like the cases shown in FIGS. 15 and 16.
However, the spring wire shown in FIG. 17 is lower in continuity of the stress dispersion in a circumferential direction than the spring wire with the same oblateness T/W=0.76 shown in FIG. 16.
That is, a conventional coil spring including flat faces provided on a spring wire with a circular section or a rectangular section has a problem that evenness of stress distribution is prevented according to intermittence of stress dispersion in a circumferential direction in a cross section.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. H06-300065    Patent Literature 2: Japanese Unexamined Patent Application Publication No. H10-82440